The answer below was true for Bootstrap 3 Release Candidates, however the stable version does include the Glyphicons library which requires a base class and an individual icon class, ex:

<span class="glyphicon glyphicon-calendar"></span>

Original answer:

From the docs:

With the launch of Bootstrap 3, icons have been moved to a separate
repository. This keeps
the primary project as lean as possible, makes it easier for folks to
swap icon libraries, and makes Glyphicons icon fonts more readily
available to more people outside Bootstrap.